From the viewpoint of improving the efficiency of the steam turbine, a steam turbine using a main stream of steam having a temperature of about 600° C. has been realized. To further improve the efficiency of the steam turbine, research and development are underway for setting a temperature of the main stream of steam to about 650 to 750° C.
Since such a steam turbine has the main stream of steam of a high temperature, it is required to use a heat-resisting alloy for some component parts. But, the heat-resisting alloy is expensive and is hardly fabricated to produce large-size parts, so that the heat-resisting alloy cannot be used for some component parts. A portion configured of such component parts might have poor material strength when the steam temperature is increased to a high level. Accordingly, as described in, for example, JP-A 2006-104951 (KOKAI), a technology of suppressing material strength from lowering due to a high temperature by cooling the component parts which have a high temperature is under study.
JP-A 2006-104951 (KOKAI) describes a technology of cooling a diaphragm outer ring by forming a diaphragm outer ring supporting a stationary blade with a cooling passage for flowing cooling steam in a steam turbine having a double-structured casing which is comprised of an outer casing and an inner casing.
Since the steam turbine has a large casing, it is desirably made of not a heat-resisting alloy but a conventionally used heat-resisting steel from the viewpoint of production costs and production. And, a conventional steam turbine provided with a double-structure casing has a diaphragm outer ring for supporting a stationary blade, which is, for example, arranged partly in contact with an inner casing, so that heat tends to be conducted from the diaphragm outer ring to the inner casing. And, the conventional structure of cooling the diaphragm outer ring is not easy to sufficiently cool the inner casing which tends to have a high temperature in the double-structure casing.